Forecasting Tornadoes in the Great Plains Part I: Ingredients Jonathan Garner
Moisture • Key physical process for the generation of substantial CAPE is differential advection of low-level moisture beneath a cool/cold midlevel airmass • The warmer the midlevel temperature is, the greater the low-level moisture must be in order to generate large CAPE • Magnitude of surface dewpoints depends on the season and pattern • For example, dewpoints in the 50s could be sufficient for mini-supercells, where cold midlevel temperatures are common • Upper 60s to low 70s dewpoints are generally necessary for a dryline outbreak due to warmer midlevel temperatures
RUC analysis sounding for a cold core midlevel low tornado event. Note surface dewpoint is in the mid 50s. From Davies (2006). OUN RAOB valid 19 May 2013 at 18Z. Significantly tornadicsupercells evolved off of a dryline two hours later along a dryline. Note surface dewpoint is in the low 70s.
Moisture • Outflow dominant supercells are less likely to produce significant tornadoes • Boundary layer moisture sufficient for surface based thunderstorms that do not produce excessive outflow is dependent, in part, on midlevel temperatures • Cooler midlevel temperatures favor less low-level mixing, and thus a more humid boundary-layer. Excessive storm outflow is less likely. • Warm midlevel temperatures promote stronger boundary layer mixing and drier RH values in the mixed layer. Excessive storm outflow is more likely.
Dry boundary layer. Mean RH over the lowest 1 km AGL is 49%. Temperature at 850 mb is 24C, 700 mb is 12C, 500 mb is -7C. Humid boundary layer. Mean RH over the lowest 1 km AGL is 83%. Temperature at 850 mb is 17C, 700 mb is 8C, 500 mb is -14C.
Elevated Mixed Layer • The elevated mixed layer (EML) originates over the Mexican Plateau and southern Rockies. • It is swept downstream across the Great Plains as west-southwesterly flow develops aloft behind a departing upper-level ridge and ahead of an upper-level trough • Steep midlevel lapse rates over the Great Plains are generated by the EML • Midlevel lapse rates are considered “steep” when > 7.0 C km-1 in the 700-500 mb layer
Elevated Mixed Layer • The EML prevents surface based storms from initiating prematurely before the generation of large CAPE can occur • Rich moisture located over the lowest few km’s surmounted by an EML/steep midlevel lapse rates yields large CAPE values • The CAPE is realized after the capping inversion located at the base of the EML is eroded or lifted
CAPE • Sufficient CAPE for tornadicsupercells is regime dependent (From Garner 2013) • Dryline MLCAPE: 1500-3000 J kg-1 • Warm Front MLCAPE: 500-2700 J kg-1 • Cold Front MLCAPE: 400-1500 J kg-1
Destabilization • Surface heating/insolation is necessary over the Great Plains warm sector • Aids in reducing CINH and generating large surface based CAPE • Incomplete surface destabilization yields messy storm structures and evolution • Character of clouds observed in visible satellite imagery will often give clues as to the degree of destabilization
Destabilization • Development of warm/hot temperatures and steep low-level lapse rates precedes storm development west of the initiation zone • Discrete supercell development is favored where axes of steep low-level lapse rates protrude into a moist/unstable boundary layer airmass • Storms form off of the low-level lapse rate axis and move downshear into the moist/unstable airmass
22 May 2004 Contours are SBCAPE. Surface wind barbs in kt.
Destabilization • Most significantly tornadicsupercell events over the Great Plains are associated with an upper-level disturbance approaching the threat area from the west • Disturbance doesn’t necessarily have to be a strong shortwave trough • Subtle disturbances have been associated with significant tornado events when rich moisture, large CAPE, and a weak CINH are in place
Destabilization • Vertical motion associated with an upper-level disturbance aids in weakening the capping inversion • Low-level flow fields often back and strengthen ahead of an approaching upper disturbance • Aids in rapidly transporting higher theta-e air toward the edge of the cap, and also strengthens low-level convergence and shear • Mid/upper-level flow strengthens in response to jet streaks associated with the upper disturbance
Vertical Wind Shear • Primary tool for analyzing vertical wind shear profile is the hodograph
Vertical Wind Shear Hodograph from SGF RAOB valid 00 UTC 5 May 2003 associated with long-track violent tornadoes. Extreme deep-layer shear and large SRH combined with a fast storm motion occurring over a wide warm sector are commonly observed for long path-length tornadoes (Garner 2007).
Vertical Wind Shear Hodograph from the Purcell, OK profiler valid 00 UTC 4 May 1999. Violent tornadicsupercells were ongoing over central Oklahoma. Note the strong speed shear in the 0-1 km layer, which yields a classic sickle shaped hodograph. From Thompson and Edwards (2000).
Vertical Wind Shear Hodograph from the FWD RAOB valid 00 UTC 16 May 2013. Strong/violent tornadoes were ongoing over northern Texas. Upper-level winds are not particularly strong (but sufficient for deep-layer shear favoring supercell development). Note the enlarged hodograph structure in the 0-3 km layer, which yielded large SRH values.
Vertical Wind Shear Two proximity hodographs for tornadic storms that formed adjacent to cold-core midlevel lows. Hodograph on the left is an early spring event from 22 UTC 10 April 2005. The hodograph on the right is a summertime example from 21 UTC 1 July 2004. From Davies (2006).
Vertical Wind Shear • Deep-layer shear should be sufficiently strong for supercells • Effective-shear or 0-6 km bulk shear >40 kt • 50-65 kt ideal for significantly tornadicsupercells • Long-lived supercells (lifespan >4 hours) are most probable when: • 0-8 km bulk shear > 50 kt • 0-3 km SRH exceeds roughly 150 m2 s-2 • Height of the LCL is < 2000 m • Synoptic/mesoscale forcing is weak and storm motion is directed along an instability axis or across a wide warm sector airmass
Vertical Wind Shear • Large SRH values are more favorable for strong mesocyclones and increased tornado potential • 0-1 km or effective-layer SRH can range from 170 m2 s-2 for large CAPE environments to 500 m2 s-2 for low CAPE environments • In general, the presence of large CAPE, strong deep-layer shear, large SRH, and LCL height <1500 m greatly increases the probability for strong/violent tornadoes in the Great Plains, especially if discrete storms occur
Vertical Wind Shear • Weak storm-relative flow at the anvil level combined with strong SRH favors rapidly evolving supercellmesocyclones and a quicker transition to HP supercell modes • Fast storm motion favors long-track tornadoes, while slow storm motion yields short-track tornadoes • For fast storm motion environments, the warm sector must be wide enough to allow surface based supercells to persist for many hours
Convective Mode • Discrete supercells, and supercells in clusters, are the preferred convective mode for strong/violent tornadoes in the Great Plains (Smith et al. 2012) • When deep-layer flow and shear is parallel to the boundary storms initiate on, storms reside in the baroclinic zone for a greater amount of time, which increases the probability for mergers and linear development • When deep-layer flow and shear is perpendicular to the initiating boundary, storms move off of the boundary more quickly, which makes discrete convective modes more likely
Conceptual model showing the initiation of storms in two different flow regimes relative to the boundary. The dark arrows represent the mean cloud-layer wind and shear vector orientations, the shading represents precipitation regions, the dotted lines are convective outflow, and the hatched areas indicate where new development is likely as convective outflows merge. From Dial et al. (2010).
Convective Mode • For dryline and cold-frontal significant tornado environments, wind profiles that favor storms moving off the boundary and into the warm sector are preferred • For warm/stationary fronts and outflow boundaries, wind profiles that allow storms to reside on the boundary are preferred in order to maximize tornado potential • However, as the convective episode evolves, storm mergers along the warm/stationary front or outflow boundary will become increasingly likely, which will eventually decrease the potential for significant tornadoes
Summary • The EML is a feature commonly present during Great Plains supercell tornado events • EML combined with a moist boundary layer is necessary for the generation of large CAPE • But the EML also caps the moist/unstable boundary layer • Therefore, processes that promote strong destabilization should be identified and targeted • If destabilization is insufficient, then 1) storms will not form, or 2) storms will be weaker, and display messy structures and evolution
Summary • Different synoptic/mesoscale regimes are associated with different magnitudes of CAPE and shear • High CAPE environments, such as those along drylines, can yield significantly tornadicsupercells with relatively weaker shear • Low to moderate CAPE environments generally require greater shear for significantly tornadic storms
Summary • Discrete surface based supercells occurring in an environment characterized by large CAPE, strong deep and low-level shear, and a humid boundary layer is most favorable for strong/violent tornadoes in the Great Plains • As synoptic-scale disturbances and their associated wind fields decrease in strength, augmentation of CAPE and shear along boundaries becomes increasingly more important for significantly tornadicsupercells
References • Bunkers, M. J., J. S. Johnson, L. J. Czepyha, J. M. Grzywacz, B. A. Klimowski, and M. R. Hjelmfelt, 2006b: An observational examination of long-lived supercells. Part II: Environmental conditions and forecasting. Wea. Forecasting, 21, 689-714. • Davies, J. M., 2006: Tornadoes with cold core 500-mb lows. Wea. Forecasting, 21, 1051-1062. • Dial, G. L., J. P. Racy, and R. L. Thompson, 2010: Short-term convective mode evolution along synoptic boundaries. Wea. Forecasting, 25, 1430-1446.
References • Garner, J. M., 2007: A preliminary study on environmental parameters related to tornado path length. Electronic J. Oper. Meteor., Paper 2007-EJ5. • Garner, J. M., 2013: A study on synoptic-scale tornado regimes. Electronic J. Severe Storms Meteor., In Press. • Smith, B. T., R. L. Thompson, J. S. Grams, C. Broyles, and H. E. Brooks, 2012: Convective modes for significant severe thunderstorms in the contiguous United States. Part I: Storm classification and climatology. Wea. Forecasting, 27, 1114-1135. • Thompson, R. L., and R. Edwards, 2000: An overview of environmental conditions and forecast implications of the 3 May 1999 tornado outbreak. Wea. Forecasting, 15, 682-699.